Inflatable occupant restraint devices for motor vehicles have been under development worldwide for many years. Gas generating compositions for inflating the occupant restraint devices have also been under development for many years and numerous patents have been granted thereon. Because the inflating gases produced by the gas generants must meet strict toxicity requirements, most, if not all gas generants now in use, are based on alkali or alkaline earth metal azides. Sodium azide is presently the preferred fuel for gas generant compositions as it reacts with oxidizing agents to form a relatively non-toxic gas consisting primarily of nitrogen.
A major problem associated with azide based gas generants is the extreme toxicity of the azide itself. The toxicity of the azide based generants makes their use inherently difficult and relatively expensive. In addition, the potential hazard and disposal problems of unfired inflation devices containing azide based generants must be considered.
In contrast, the non-azide based gas generants (i.e., 5-aminotetrazole) provide significant advantages over the azide based gas generants with respect to hazards during manufacture and disposal. Unfortunately, a number of non-azide based gas generants heretofore known produce unacceptably high levels of undesirable substances upon combustion (i.e., toxic gases and particulates). The most difficult undesirable gases to control are the various oxides of nitrogen (NO.sub.x) and carbon monoxide (CO). Typical non-azide gas generants require the use of oxidizers such as strontium nitrate, sodium nitrate, potassium nitrate and the like to achieve a burn rate that produces a significant amount of gas in the required time period.
The reduction of the level of undesirable gases upon combustion of non-azide gas generants and a reduction of the formation of solid combustion particles (slag) requires a special combination of materials. For instance, manipulation of the oxidizer/fuel ratio reduces either the NO.sub.x or CO. More specifically, increasing the ratio of oxidizer to fuel minimizes the CO content upon combustion because the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, this approach results in increased amounts of NO.sub.x. The relatively high levels of NO.sub.x and CO produced upon combustion of non-azide gas generants and the difficulty presented in forming easily filterable solid combustion products is due, in part, to the relatively high combustion temperatures exhibited by the conventional non-azide gas generants. Utilizing lower energy fuels to reduce the combustion temperature is ineffective because the lower energy fuels do not provide a sufficiently high rate of gas generation, or burn rate, for use in vehicle restraint systems. Adequate burn rate of the gas generant is required to ensure that the airbag system will operate readily and properly.
The aforementioned problems are solved by the present invention, which discloses gas generants that contain from 35-80 wt. % PSAN, from 15-30 wt. % non-azide fuel and 0.5-7.0 wt. % metallic silicon. The generant of the invention may also contain iron oxide at up to 7.0 wt. % and an organic binder at up to 5.0 wt. %. The gas generants of this invention produce low levels of easily filterable combustion products and rapidly produce inflating gases in sufficient quantities with a minimum production of undesired gases. More preferably, this invention relates to non-azide based gas generants that contain up to about 75 wt. % PSAN, up to about 3 wt. % metallic silicon and up to about 3 wt. % iron oxide.
U.S. Pat. No. 3,912,562 discloses a gas generating composition which comprises a fuel such as a carbonaceous material, aluminum or magnesium; an inorganic oxidizer such as metal chlorates, metal perchlorates and ammonium nitrate; and a coolant such as magnesium hydroxide and/or magnesium carbonate.
U.S. Pat. No. 5,583,315 discloses a smoke free propellant containing 40-85 wt. % AN, 4-40 wt. % of a binder, 0-40 wt. % of an energetic plasticizer and 0.1-8.0 wt. % of a reinforcing agent.
U.S. Pat. No. 5,035,757 discloses a gas generating mixture useful for inflating an automobile crash bag, the pyrotechnique mixture comprising: (1) a fuel selected from the azole compounds; (2) an oxygen containing oxidizer; (3) a high temperature slag forming material selected from a group consisting of alkaline earth metal oxides, hydroxides, carbonates and oxalates; and (4) a low temperature slag forming material selected from the group consisting of silicon dioxide, boric oxide, alkaline metal silicates and naturally occurring clays and talcs.
U.S. Pat. No. 5,139,588 discloses a gas generating composition comprising: (1) a non-azide fuel; (2) an oxygen containing oxidizer; (3) an alkaline earth metal salt of an inorganic or organic acid such as 5-aminotetrazole; and (4) a low temperature slag forming material selected from clays, talcs and silica.
U.S. Pat. No. 5,531,941 discloses gas generant compositions comprising triaminoguanadine and phase stabilized ammonium nitrate. This patent also discloses a process for the preparation of such compositions.
U.S. 5,386,775 discloses an azide-free gas generant composition that contains a low energy nitrogen containing fuel combined with a burn rate accelerator comprising alkali metal salts of organic acids. Examples of a low energy nitrogen containing fuel are ammonium oxalate, glycine nitrate and azodicarbonamide. This patent also provides examples of organic acids as tetrazoles, triazoles, 5-aminotetrazole, 5-nitroaminotetrazole and bitetrazoles. This patent does not suggest nor disclose the use of a PSAN based oxidizer system in combination with 0.5 to 7 wt. % silicon.
U.S. Pat. No. 5,516,377 discloses a gas generating composition comprising 5-nitraminotetrazole and an oxidizer selected from metal oxides, inorganic nitrates, metal peroxides, metal hydroxides and mixtures thereof.
U.S. Pat. No. 5,507,891 discloses propellant compositions which function with hybrid inflator systems. The propellant composition of this reference comprises a fuel such as cyclotrimethylenetrinitramine at 40-80 wt. %; an oxidizer such as ammonium nitrate at 0-35 wt. % and an inert or energetic binder at 0-15 wt. %
U.S. Pat. No. 5,500,061 discloses the addition of silicon (Si) powder at about 0.4-6 wt % to unstabilized ammonium nitrate propellant formulations to increase the performance specific impulse. The formulations of this reference are designed for rocket motors and utilize energetic binders such as glycidyl azide polymers. Further, the compositions disclosed in this patent have specific impulse values of 234-250 seconds at 6895 kPa (1000 psi) motor operating pressure. In contrast, the gas generants of the present invention have specific impulses less than 225 seconds at 6895 kPa (1000 psi). In addition, Warren uses a castable urethane binder system which presents toxicity problems and increased costs in vehicle restraint systems.
U.S. Pat. No. 4,111,728 discloses a castable gas generator composition comprising 25-40 wt. % of a binder of polyether or polyester and 45-60 wt. % ammonium nitrate coated with a compound selected from the group consisting of magnesium oxide and magnesium nitrate; and an effective amount of a burn rate modifier.
U.S. Pat. No. 5,596,168 and U.S. Pat. No. 5,589,661 disclose a solid propellant for rocket propulsion systems or gas generants that comprises 35-80 wt. % of a phase stabilized ammonium nitrate; 15-50 wt. % of a high energy binder system containing an energy rich plasticizer and 0.2-5 wt. % of burn rate modifier selected from vanadium oxide and molybdenum oxide.
U.S. Pat. No. 4,158,583 discloses a high performance rocket propellant with greatly reduced hydrogen chloride emissions. The propellant comprises a hydrocarbon binder at 10-15 wt. %, ammonium nitrate (AN) at 40-70 wt. %; 5-25 wt. % of a powdered metal fuel such as aluminum and 5-25 wt. % of ammonium perchlorate.
AN contains no halogens, burns without smoke production and is less toxic than other conventionally employed oxidizing materials. AN is, other than ammonium perchlorate, one of the few readily available, inexpensive, inorganic oxidizers useful in energetic applications. AN is also the only inorganic oxidizer which will burn completely to a non-toxic gas, leaving no solid residue.
However, the attractiveness of current commercially available ammonium nitrate in energetic compositions is tempered by several severely limiting drawbacks. Such drawbacks include an energetic performance significantly lower than comparable ammonium perchlorate-based compositions, low burning rates at relatively high pressures compared to other oxidizer-containing compositions, and greater hygroscopicity (moisture sensitivity) than ammonium perchlorate.
Also, ammonium nitrate is thermally unstable. AN passes through five distinct crystal phase changes from about -17.degree. C. to 169.degree. C. The most disadvantageous change or transition is the Phase IV.revreaction.Phase III transition, occurring at about 32.3.degree. C. This Phase IV.revreaction.Phase III transition is characterized by a significant irreversible increase in crystal volume. Thus, repeated cycling of ammonium nitrate-based pyrotechnique compositions through the Phase IV to Phase III transition temperature is known to cause growth of the grain and destruction of grain integrity. The result is an increased porosity and loss in mechanical strength which is highly undesirable in energetic composition.
Over the years, numerous efforts to stabilize ammonium nitrate to prevent or sufficiently suppress the Phase IV.revreaction.Phase III transition have been made. In the agrochemical field, a wide variety of ingredients have been tried at one time or another to prevent caking. In the energetic composition field, efforts to stabilize AN have included combining ammonium nitrate with materials such as potassium nitrate, zinc oxide, magnesium oxide, potassium fluoride and nickel oxide. Certain lithium, calcium, barium, aluminum salts and other metal salts of the nitrate anion have also been used. Further, compounds such as urea, ethylene diamine nitrate, diethylene triaminotrinitrate, guanidinium nitrate, silicates, and for instance, melamine have also been investigated as ammonium nitrate stabilizers.
The following patents disclose various techniques to produce a phase stabilized ammonium nitrate: U.S. Pat. No. 5,292,387; U.S. Pat. No. 4,001,377; U.S. Pat. No. 4,124,368; U.S. Pat. No. 4,552,736; U.S. Pat. No. 4,925,600; U.S. Pat. No. 5,098,683; U.S. Pat. No. 2,590,054; U.S. Pat. No. 2,657,977; U.S. Pat. No. 2,943,928; U.S. Pat. No. 3,212,944; and U.S. Pat. No. 3,428,418.
EP 0689527B1 relates to ammonium nitrate stabilized with certain metal dinitramide salts. This reference teaches that a dinitramide salt such as potassium dinitramide is present in the mixture at levels of from 5-25 wt. %. The propellant compositions using the stabilized AN include metal fuels such as aluminum, boron, magnesium and the like; a suitable binder; and a ballistic catalyst such as aluminum oxide or zirconium carbide.
None of the above discussed references disclose gas generant compositions which will function satisfactorily in airbag inflator systems. The required need of high burn rates, low toxicity of combustion gases, reduced particulate production and reduced tendency to self-extinguish is accomplished through the novel and unobvious formulation of this invention.